This application addresses broad Challenge Area (15) Translational Science and specific Challenge Topic 15-DK-106: Translating basic hematology concepts. Haploinsufficiency of ribosomal protein (rp) genes underlies the inherited marrow failure syndromes Diamond-Blackfan anemia (DBA) and the 5q- myelodysplastic syndrome (5q- MDS). Although it is known that deficiencies in ribosomal protein expression result in disrupted ribosome biogenesis, the molecular mechanisms by which such defects lead to marrow failure and MDS are unclear. This lack of understanding limits our ability to discern the mechanism of action of current therapies and hampers the development of new therapies for these disorders. It is accepted that the ability of cells to progress through the cell cycle is tightly linked to nascent ribosome biogenesis. The upregulation of nascent ribosome biogenesis serves two major functions: (i) it provides the increased translational capacity required for a cell to achieve the critical mass to enter the cell cycle, and (ii) it ensures that each daughter cell obtains the appropriate number of ribosomes. Given that a change in the ratio of ribosomes to mRNA would eventually modify the pattern of translation and the genetic program, resulting in aberrant forms of growth, evidence is mounting that checkpoints exist to sense and respond to the status of nascent ribosome biogenesis. Indeed, earlier studies had suggested a role for the p53 tumor suppressor gene, and more recent results show that p53 is activated in response to deficiencies in the production of rps, leading to cell- cycle arrest due to the binding and inhibition of human double minute 2 (HDM2) by 60S rpL11 and rpL5. It has been demonstrated that this checkpoint response is mediated by rpL11 and rpL5 by distinct mechanisms, depending on whether a 40S or 60S rp is depleted. The hypothesis that will be tested in this study is that the rpL11/rpL5 checkpoint response to rp stress plays a causal role in hematopoietic failure in DBA and 5q- MDS and that current medical therapies target this pathway for therapeutic gain. To address this hypothesis, the following aims will be carried out: Aim 1: Characterize rpL5 and rpL11 translation status in human hematopoietic cells depleted of either small or large ribosomal subunit proteins;Aim2: Determine the effects of dexamethasone, lenalidomide, and leucine on the p53 response and on rpL5 and rpL11 translation in primary human hematopoietic cells depleted of ribosomal proteins;and Aim 3: Assess the effects of dexamethasone, lenalidomide, and leucine in an in vivo model of ribosomal protein deficiency and validate those findings in phenotyped patient samples. The knowledge derived from these studies will be critical for identifying novel therapeutic targets and for the development of new rationally designed therapies. PUBLIC HEALTH RELEVANCE: Mutations in ribosomal protein genes affect the production of ribosomes, the machinery inside the cell which is responsible for the synthesis of proteins, resulting in disorders of the hematopoietic system, such as anemia, a defect in the production of red blood cells. Although therapies are available for these diseases, their mechanism of action is not known. It is likely that the anemia of patients with mutations in genes for ribosomal proteins is due to the fact that defects in the production of ribosomes cause an arrest in cell division which can result in cell death. The aim of the proposed project is to determine the effects of therapies used to treat anemias caused by mutations in ribosomal protein genes and to identify mechanisms that trigger arrest in cell proliferation and cell death when ribosome production is impaired. This will allow for the identification of the cellular processes that impair production of red blood cell and will be essential in the design of new therapies.